Silver catalysts in the form of granules, gauze, wire turnings, crystals, and the like have been used for many years to produce formaldehyde by the oxidative dehydrogenation of methanol. Silver crystals are especially suitable for this purpose, since they are very selective and have little tendency to promote side reactions and the formation of by-products under reaction conditions which permit high overall yields.
Numerous prior art patents and publications describe the addition of small amounts of promoters to silver catalysts used in the production of formaldehyde from methanol. For example, U.S. Pat. No. 1,937,381, issued to Bond et al on Nov. 28, 1933 and assigned to du Pont, describes silver crystal oxidation catalysts containing promoters such as oxides of tungsten, vanadium, cerium, thorium, molybdenum, chromium, aluminum and zinc. Silver-cadmium alloy catalysts for the oxidative dehydrogenation of methanol are shown in U.S. Pat. No. 3,334,143, issued to Stiles on Aug. 1, 1960 and also assigned to du Pont. Silver containing up to 10% of an oxide of barium, strontium or calcium and up to 8% of an oxide of indium are taught as methanol oxidative dehydrogenation catalysts in U.S. Pat. No. 4,045,369, issued to Cantaluppi on Aug. 30, 1977 and assigned to S.A.E.S. Getters S.p.A. None of these patents suggest the use of lead in combination with silver as a methanol oxidative dehydrogenation catalyst.
Two literature articles from the Soviet Union, each entitled "Catalytic Properties of Silver Alloys in the Conversion of Methanol into Formaldehyde", also disclose various additives alloyed with silver for the oxidation of methanol to formaldehyde. The first is an abstract of The Russian Journal of Physical Chemistry, 45 (10), 1971 p. 1524. It discloses as methanol oxidative dehydrogenation catalysts silver alloys of aluminum, magnesium, copper, zinc, gallium, germanium, selenium, cadmium, indium, tin, antimony, tellurium and bismuth. The other publication is an article from Tekh. Progr. Dostizh Nauki Khim, Prom. 1973, p. 191-5 abstracted in Chemical Abstracts Vol. 81, 68971, 1974. It describes the same silver alloys as the first abstract except that the selenium and antimony alloys are not included. There is no mention of lead-silver catalysts in these articles. Of specific interest, however, is the description of the attempt to improve silver catalysts by adding tin and germanium (or the same group in the Periodic Table as lead) to improve the oxidative dehydrogenation of methanol to formaldehyde. Both of these additives to silver gave exceptionally poor results for formaldehyde production, compared to the use of silver alone.
A paper by Schwab entitled "Metal Electrons and Catalysts" in the The Transactions of the Faraday Society Vol. 42, 1946 p. 689-697, describes heterogeneous and homogeneous alloys of lead-silver, among other alloys. However, these catalysts were used to dehydrogenate formic acid to hydrogen and carbon dioxide, a reaction substantially different from the oxidative dehydrogenation of methanol to formaldehyde.
Another prior art patent, U.S. Pat. No. 3,948,997, issued Apr. 6, 1976 to Howe et al, describes a process for the vapor phase oxidation of .alpha.,.beta.-diols such as ethylene glycol to .alpha.,.beta.-diones such as glyoxal at elevated temperatures in the presence of a catalyst containing as essential constituents, one or more metals of Group Ib of the Periodic Table comprising copper, silver and gold, and one or more elements from Group IVa, comprising germanium, tin and lead, and Group Va, comprising nitrogen, phosphorus, arsenic, antimony and bismuth. However, there is no specific disclosure in this patent of a lead-silver catalyst, per se, nor is there any suggestion that any of the catalysts disclosed generically or specifically, could be used for the oxidative dehydrogenation of methanol to formaldehyde.